Estimation of soil water retention curves from soil bulk electrical conductivity and water content measurements

Measurement of soil water retention curves (SWRCs) is time consuming, and there is no single laboratory device available to measure a SWRC over an entire range of relevant pressures. The van Genuchten (vG) model is commonly used to characterize the shape of the SWRC. Bulk soil electrical conductivity as a function of water content, sigma(theta), has been used to estimate hydraulic properties of unsaturated soils, thus making it possible to relate sigma(theta) and SWRC. The saturated and residual water content values, theta(s) and theta(r), can be estimated from soil bulk density and particle size distribution. In this study, we present an approach to estimate vG parameters m and alpha from sigma measured at saturated and residual soil water contents, as well as sigma(theta) values measured at intermediate water contents. A thermo-time domain reflectometry (thermo-TDR) sensor is used to measure sigma and theta of the same soil sample volume. SWRCs for three soils (Glassil 530 sand, Tennessee silt loam and Illinois clay loam with bulk densities ranging from 1.52 to 1.67 g cm(-3), 1.05 to 1.25 g cm(-3) and 1.05 to 1.2 g cm(-3), respectively) are estimated from sigma(theta) measurements and compared with direct SWRC measurements obtained with a tension table and pressure plate extractors. Additional comparisons are made using data obtained from the literature. The proposed method to estimate SWRCs performs well when compared to direct SWRC measurements (with an average RMSE and an average bias of 0.041 cm(3) cm(-3) and 0.008 cm(3) cm(-3), respectively). Results indicate that the new sigma(theta) based method accurately estimates SWRCs.

Automated summary

The study introduced a new method to estimate soil water retention curves parameters using soil electrical conductivity and water content, showing that the new method accurately estimates SWRCs compared to direct measurements.